1. Field of the Invention
The present invention relates to a battery protection IC and a battery device, and more particularly, to protection of an IC when a charger is connected to a battery device in a reverse direction.
2. Description of the Related Art
Nowadays, portable electronic devices have become widespread, and those devices are driven by a battery device. The battery device includes a secondary battery and a battery protection IC incorporating a protection circuit for controlling charge/discharge of the secondary battery. The battery protection IC has the function of controlling the charge/discharge of the secondary battery and also the function of protecting the secondary battery from an overcharge, an overdischarge, and an overcurrent. Further, the battery protection IC has another function of protecting the secondary battery and an IC even when a charger for charging the secondary battery is connected with reverse polarity to that in normal connection.
FIG. 2 is a block diagram illustrating a battery protection IC 3 and a battery device 1.
The battery device 1 includes a secondary battery 2, the battery protection IC 3, a discharge control FET 4 and a charge control FET 5 serving as switches, a capacitor 6, an input resistor 7, a current limiting resistor 8, and external terminals 11 and 12 to be connected to a charger 13 or a load. The battery protection IC 3 includes an overdischarge detection circuit 31, an overcharge detection circuit 32, an overcurrent detection circuit 33, a control circuit 34, a VDD terminal 15, a VSS terminal 16, a CO terminal 17 for charge control, a DO terminal 18 for discharge control, and a VM terminal 19 for overcurrent detection.
The secondary battery 2 has a positive terminal connected to the VDD terminal 15 of the battery protection IC 3 via the input resistor 7, and a negative terminal connected to the VSS terminal 16 of the battery protection IC 3. The capacitor 6 is connected to the VDD terminal 15 and the VSS terminal 16 of the battery protection IC 3. The discharge control FET 4 and the charge control FET 5 are connected in series between the negative terminal of the secondary battery 2 and the external terminal 12 of the battery device 1. The discharge control FET 4 has a gate connected to the discharge control terminal DO of the battery protection IC 3. The charge control FET 5 has a gate connected to the charge control terminal CO of the battery protection IC 3. The discharge control FET 4 and the charge control FET 5 are each provided with a gate oxide film protection diode between its gate and source. The current limiting resistor 8 is connected between the VM terminal 19 of the battery protection IC 3 and the external terminal 12.
The overdischarge detection circuit 31 and the overcharge detection circuit 32 each have input terminals connected to the VDD terminal 15 and the VSS terminal 16, and an output terminal connected to the control circuit 34. The overcurrent detection circuit 33 has input terminals connected to the VM terminal 19 and the VSS terminal 16, and an output terminal connected to the control circuit 34 (see, for example, Japanese Patent Application Laid-open No. 2009-177937).
FIG. 3 is a circuit diagram illustrating parasitic diodes of the conventional battery protection IC 3.
The parasitic diodes of the battery protection IC 3 are typically parasitic capacitances of transistors in the circuit. For example, a parasitic diode D1 is present between the VM terminal 19 and the VDD terminal 15. Further, parasitic diodes D2, D3, and D4 are present between the CO terminal 17 and the VDD terminal 15. Still further, parasitic diodes D5 and D6 are present between the DO terminal 18 and the VDD terminal 15.
The charger 13 outputs a high voltage of about 30 V. The charger 13 is connected so that a high potential is connected to the external terminal 11 side of the battery device 1 and a low potential is connected to the external terminal 12 side of the battery device 1.
When the charger 13 is connected to the battery device 1 in the reverse direction, an abnormal current is generated in the battery device 1 in the following current paths.
First, the parasitic diode present between the VM terminal 19 and the VDD terminal 15 is connected in the forward direction, and a current flows in a path from the charger 13 to the current limiting resistor 8, the VM terminal 19, the parasitic diode D1 (D3 and D4), the input resistor 7, and the charger 13 in this order.
Next, a gate oxide film protection diode of the charge control FET 5 operates so as to clamp a potential difference between the gate and the source of the charge control FET 5. Accordingly, the parasitic diode connected to the CO terminal 17 of the battery protection IC 3 is connected in the forward direction, and a current flows in a path from the charger 13 to the gate oxide film protection diode of the charge control FET 5, the CO terminal 17, the parasitic diode D4, the input resistor 7, and the charger 13 in this order.
When those currents flow, a voltage is generated across the input resistor 7, and hence a voltage exceeding a rated voltage is applied between VDD and VSS of the battery protection IC 3.
In this case, the current limiting resistor 8 connected to the VM terminal 19 of the battery protection IC 3 suppresses a current when the charger 13 is connected in the reverse direction. Further, a resistor R1 provided in the path from the CO terminal 17 to the VDD terminal 15 also suppresses a current when the charger 13 is connected in the reverse direction. In this way, the current limiting resistors are provided in the internal current paths, to thereby suppress the current when the charger 13 is connected in the reverse direction.
However, the battery protection IC 3 and the battery device 1 described above need the current limiting resistor 8, and hence there is a problem that the number of external components of the battery protection IC 3 increases.
Further, the internal current limiting resistor has a tradeoff relationship to the influence on the normal operation of the battery protection IC 3, and hence there is a problem that the resistance value thereof cannot be increased unnecessarily.